And let �s not ofrget about the ambiguity the different style that Americans and Brits use with the dates...For Europeans, the date you wrote is April 9th, but for most Americans (correct me if I �m wrong) the month goes first, so 9/4 would actually be Septemember 4th.
So I always tell my students to use the format 9th April, or April 9th, to avoid that.
As far as I know for informa letters, the following formats are acceptable:
9/4/ 2015
9 Apr 2015
or 9th April or April 9th
you can even omit the year I think
For semi-formal letters, you never omit the year,
and for formal ones, I dinstictly remember this being the most acceptable:
Thursday, 9th April 2015.